Bug 1610334 - add accessible labels to close/unpip buttons, r=mconley,fluent-reviewers,flod

Differential Revision: https://phabricator.services.mozilla.com/D62460

--HG--
extra : moz-landing-system : lando

toolkit/components/pictureinpicture/content/player.xhtml

@@ -21,10 +21,10 @@
       <xul:browser type="content" primary="true" remote="true" remoteType="web" id="browser" tabindex="-1"></xul:browser>
     </div>
     <div id="controls">
-      <button id="close" class="control-item" tabindex="3"></button>
+      <button id="close" class="control-item" data-l10n-id="pictureinpicture-close" tabindex="3"></button>
 
       <div id="controls-bottom">
-        <button id="unpip" class="control-item" tabindex="2"></button>
+        <button id="unpip" class="control-item" data-l10n-id="pictureinpicture-unpip" tabindex="2"></button>
         <div class="gap"></div>
         <button id="playpause" class="control-item" tabindex="1" autofocus="true"
                 data-l10n-id="pictureinpicture-pause"/>

toolkit/locales/en-US/toolkit/pictureinpicture/pictureinpicture.ftl

@@ -14,3 +14,9 @@ pictureinpicture-mute =
 pictureinpicture-unmute =
   .aria-label = Unmute
 
+pictureinpicture-unpip =
+  .aria-label = Send back to tab
+
+pictureinpicture-close =
+  .aria-label = Close
+